Photonic crystals are one of the most significant recent advances in the field of optical devices and optical signal processing. A photonic crystal possesses a photonic band gap that defines a range of electromagnetic frequencies that are unable to propagate in the crystal. Photonic crystals include periodically-arranged regions of one dielectric material within a surrounding dielectric material. The precise details and dimensionality of the periodic arrangement, along with the refractive index contrast between the periodically arranged regions and the surrounding material, dictate the characteristics of the photonic band gap of a photonic crystal. Important material design considerations include the size, spacing and arrangement of the periodically-arranged dielectric regions within a volume of surrounding material as well as the refractive indices of the periodically-arranged dielectric regions and the surrounding material. The periodicity of the periodically-arranged dielectric media can extend in one, two or three dimensions. These considerations influence the magnitude of the photonic band gap, the frequency range of light or other electromagnetic energy (e.g. infrared, microwave etc.) that falls within the photonic band gap and whether the photonic band gap is full (in which case the photonic band gap effect is manifested regardless of the direction of propagation of the incident light) or partial (in which case the photonic band gap effect is manifested for some, but not all, directions of propagation).
Light having an energy within the photonic band gap and propagating in a direction defined by the photonic band gap is blocked and unable to propagate in a photonic crystal. When external light having an energy and direction of propagation within the photonic band gap is made incident to a photonic crystal, it is unable to propagate through the crystal. Instead, it is perfectly reflected. Light with an energy or direction of propagation outside of the photonic band gap, on the other hand, passes through a photonic crystal.
Effects analogous to doping or defects in semiconductors may also be realized in photonic crystals to further control the interaction of photonic crystals with light. The periodicity of photonic crystals can be perturbed in ways analogous to the introduction of dopants and defects in semiconductors. The periodicity of a photonic crystal is a consequence of a regular and ordered arrangement of macroscopic dielectric regions or media (e.g. rods or holes) within a surrounding medium (e.g. dielectric slab). Effects that interrupt the arrangement of macroscopic dielectric media can be used to break the periodicity to create photonic states within the photonic band gap. Possible ways of perturbing an array of rods in a surrounding dielectric slab, for example, include varying the size, position, optical constants, chemical composition of one or more rods or forming rods from two or more materials. The ability to create photonic states within the photonic band gap provides further flexibility in controlling the frequencies and directions of incident light that are reflected, redirected, localized or otherwise influenced by a photonic crystal.
By introducing defects into photonic crystals, it is possible to control the direction of propagation of light and to confine light. The introduction, for example, of a linear defect in a quasi-two-dimensional photonic crystal confines light and permits use of the photonic crystal as a waveguide for wavelengths within the photonic band gap of the crystal. Point defects can be used to localize light and to form resonant cavities. Examples of photonic crystals and the effect of defects in photonic crystals on the properties of propagating light can be found in the publications: “Linear waveguides in photonic-crystal slabs” by S. G. Johnson et al. and published in Physical Review B, vol. 62, p. 8212-8222 (2000); “Photonic Crystals: Semiconductors of Light” by E. Yablonovich and published in Scientific American, p. 47-55, December issue (2001); Photonic Crystals: Molding the Flow of Light ; by J. D. Joannopoulos et al., Princeton University Press (1995); and “Channel drop filters in photonic crystals” by S. Fan et al. and published in Optics Express, vol. 3, p. 4-11 (1998).
It is widely expected that photonic crystals will be significant components in the next-generation information, optical and communication systems. Many people believe that the potential ability to control the propagation of light offered by photonic crystals may exceed the ability of semiconductors to control the propagation of electrons and that a commensurately greater economic benefit will result from the development of new technologies and industries based on photonic crystals and their ability to selectively inhibit, direct or localize the propagation of light in increasingly complex ways. The technological areas in which photonic crystals are projected to make an impact continue to grow in scope. Projected applications include Leeds and lasers that emit light in very narrow wavelength ranges or that are of baroscopic dimensions, direction selective reflectors, narrow wavelength optical filters, micro cavities for channeling light, color pigments, high capacity optical fibers, integrated photonic and electronic circuits that combine photonic crystals and semiconductors to produce new functionality, devices for light confinement, optical switches, modulators, and miniature waveguides.
In order to realize the potential for photonic crystals in integrated optical systems, it is necessary to devise ways to efficiently couple light into photonic crystals. Efficient coupling from conventional fibers and waveguides to photonic crystals and vice versa is one desired objective. In the case of photonic crystals having defects, it is further desirable to develop a capability for the direct coupling of light from a waveguide or other interconnect into the defect. Another important objective is the efficient coupling of light from one photonic crystal to another and from a photonic wire (or other waveguide) to a photonic crystal (and vice versa).
U.S. patent application Ser. No. 10/855,482 ('482 application) filed by the instant assignee describes a low loss method for the coupling of light from an optical fiber to a slab waveguide. The '482 application provides devices and a general framework for achieving improved coupling efficiency between elements of an optical circuit that differ in physical size or cross-section and/or refractive index. Improved coupling efficiency is achieved through a coupling device that maintains or approximately maintains the impedance encountered by a propagating optical signal as the geometric cross-section and/or refractive index in the direction of propagation varies over a finite distance. The '482 application recognizes that impedance variations that occur along the direction of propagation lead to losses in the transmission of an optical signal and presents devices in which competing geometric and constitutive influences on impedance can be balanced to minimize variations in impedance so that transmission efficiency can be improved.
U.S. patent application Ser. No. 11/124,736 ('736 application) filed by the instant assignee extends the impedance matching concept presented in the '482 application to photonic crystals and waveguides. The '736 application specifically provides for the efficient coupling of an optical signal to or from a photonic wire waveguide to a photonic crystal waveguide or defect. By tailoring the shape of a photonic wire waveguide in a way that conforms to changes in refractive index as the signal enters a photonic crystal or waveguide, it becomes possible to maintain constant or approximately constant impedance and to minimize losses upon transfer of the signal from the photonic wire waveguide to a photonic crystal waveguide or defect.
U.S. Pat. No. 6,859,304 ('304 patent) granted to the instant assignee describes a photonic crystal and channel drop filter that comprises a switchable chalcogenide component. The chalcogenide material can be reversibly transformed into a plurality of structural states that possess distinct optical constants. When included in a defect in a photonic crystal, the chalcogenide material provides for tunable functionality due to the ability to reversibly vary its refractive index and absorption coefficient through control of its structural state. In the case of a photonic crystal resonator, the cavity can be made absorptive or non-absorptive through proper selection of the structural state of the chalcogenide. This feature can be exploited, for example, to produce a channel drop filter that can be switched on or off at will to control the routing of light in photonic crystals and optical integrated circuits in general.
The '482 application, the '736 application and the '304 patent provide devices and methods for minimizing losses during the transfer of an optical signal between photonic crystals, cavities and other defects of photonic crystals, photonic wire waveguides, slab waveguides, and channel drop filters and provide effective strategies for optimizing the efficiency of the routing and processing of optical signals at the device level in photonic integrated circuits. To further advance the field of photonic integrated circuits, it is desirable to develop systems and processing methods that simplify the integration of conventional fibers and waveguides with photonic crystals, waveguides, and planar structures in general.
An important objective is the realization of photonic integrated circuits through economically feasible manufacturing methods such as planar fabrication processes. A key objective is the fabrication of planar photonic integrated circuit elements that can be readily and efficiently interconnected to conventional optical fibers. Optical fibers represent the wiring of all-optical networks and are the medium of choice for transmitting optical signals over long distances. In order to improve the commercial viability of photonic integrated circuits, it is desirable develop optical platforms that include planar photonic devices (active or passive) and are readily joined with conventional fibers. Key issues include alignment of the fiber core with a planar waveguide or planar photonic crystal and minimization of transfer losses at the junction between the optical fiber and planar structure.